The present disclosure relates to a reception apparatus, a reception method, and a program. More particularly, the disclosure relates to a reception apparatus, a reception method, and a program for performing channel estimation accurately when the delay amount of echo is significant or when it is difficult to distinguish a dominant wave from echo.
There exists the DTMB (Digital Terrestrial Multimedia Broadcast) standard for terrestrial digital broadcasts. According to the DTMB standard, either a single carrier modulation system or a multi-carrier modulation system may be selected as the data modulation system.
In the description that follows, single carrier transmission will refer to the act of transmitting data by the single carrier modulation system, and multi-carrier transmission will signify the act of transmitting data by the multi-carrier modulation system.
Upon single carrier transmission under the DTMB standard, a PN signal and a data signal are sent periodically for data transmission. Upon multi-carrier transmission, an IFFT (Inverse Fast Fourier Transform) operation is performed on the PN signal and data signal, and the resulting data is transmitted periodically. The PN signal is a known signal that includes a predetermined data sequence. This signal is added to each frame as a guard interval against inter-frame interference.
FIG. 1 is a schematic view showing the frame format of the DTMB standard.
As shown in FIG. 1, each frame under the DTMB standard is made up of a single PN signal (PN) and a single data signal (DATA). The frame length is defined as the PN length plus the data length. The frame length is shown in FIG. 2. The PN length is given as the number of symbols (selectable from 420, 595, or 945 symbols). The data length is fixed to 3780 symbols (i.e., 3780 samples following the IFFT operation).
If the PN length is 420 symbols (in the case of PN420), the frame length is 4200 symbols; if the PN length is 595 symbols (PN595), the frame length is 4375 symbols; if the PN length is 945 symbols (PN945), the frame length is 4725 symbols.
A reception apparatus determines the PN length of the PN signal used in a received signal by reproducing a PN sequence (i.e., the same data sequence as that included in each PN signal of 420, 595, or 945 symbols) and by finding correlation values between the PN sequence and the received signal. The reception apparatus proceeds to receive a data signal subsequent to the PN signal and perform various processes such as equalization on the received data signal.
Incidentally, sparse equalization is one typical technique of equalization. This is a technique for making it possible to equalize long-delayed echo by inserting a variable delay buffer in the data line of a filter used by an equalizer so as to virtually prolong the tap length.
The reception apparatus that complies with the DTMB standard is furnished with two equalizers: one for receiving the data transmitted by single carrier transmission, and another for receiving data sent by multi-carrier transmission.
FIG. 3 is a block diagram showing a structure of a single carrier equalizer that equalizes the signal representative of the data transmitted by single carrier transmission.
Circuits located upstream of the single carrier equalizer frequency-convert the received signal into an IF signal and carry out such processes as analog to digital (A/D) conversion and orthogonal demodulation on the IF signal. The processes turn the IF signal into an input signal ID(t) signal composed of a PN signal and a data signal per frame. The input signal ID(t) is input to an FFE (Feed Forward Equalizer) 11, an LMS (Least Mean Square) operation portion 16, and a channel estimation portion 18. The single carrier equalizer equalizes time domain signals using the FFE 11 and an FBE (Feed Back Equalizer) 14.
The FFE 11 is composed of a variable coefficient filter that performs a convolution operation on the input signal ID(t) and on a coefficient C0(n), the coefficient being obtained by the LMS operation portion 16. The FFE 11 outputs to an addition portion 12 a signal OD0(t) representing the result of the convolution operation. The output signal OD0(t) of the FFE 11 is defined by the following expression (1):
                              OD          ⁢                                          ⁢          0          ⁢                      (            t            )                          =                              ∑                          i              =              0                                                      N                ⁢                                                                  ⁢                _                ⁢                                                                  ⁢                FFE                            -              1                                ⁢                                    ID              ⁡                              (                                  t                  -                  i                                )                                      ×            C            ⁢                                                  ⁢            0            ⁢                          (              i              )                                                          (        1        )            where, N_FFE denotes the tap count of the FFE 11.
The addition portion 12 adds up the output signal OD0(t) of the FFE 11 and an output signal OD1(t) of the FBE 14 to generate an equalized signal OD(t) (OD(t)=OD0(t)+OD1(t)) that is output. The equalized signal OD(t) from the addition portion 12 is output outside the single carrier equalizer and supplied to a hard decision portion 13 and an error calculation portion 15.
The hard decision portion 13 performs a hard decision on the equalized signal OD(t) fed from the addition portion 12, and outputs a signal OD′(t) representing the result of the hard decision. The signal OD′(t) is sent to the FBE 14, error calculation portion 15, and an LMS operation portion 17.
The FBE 14 is also composed of a variable coefficient filter that performs a convolution operation on the signal OD′(t) fed from the hard decision portion 13 and on a coefficient C1(n), the coefficient being acquired by the LMS operation portion 17. The FBE 14 outputs the signal OD1(t) representing the result of the convolution operation. The output signal OD1(t) is sent to the addition portion 12 that uses the signal in the addition involving the output signal OD0(t). The output signal OD1(t) of the FBE 14 is defined by the following expression (2):
                              OD          ⁢                                          ⁢          1          ⁢                      (            t            )                          =                              ∑                          i              =              0                                                      N                ⁢                                                                  ⁢                _                ⁢                                                                  ⁢                FBE                            -              1                                ⁢                                                    OD                ′                            ⁡                              (                                  t                  -                  α                  -                  i                                )                                      ×            C            ⁢                                                  ⁢            1            ⁢                          (              i              )                                                          (        2        )            where, N_FBE represents the tap count of the FBE 14, and α denotes the delay involved until the signal OD′(t) is obtained from the equalized signal OD(t).
The data line of the FBE 14 is furnished with a variable delay buffer that supports sparse equalization where the delay amount of echo is significant. The channel estimation portion 18 establishes the delay amount “delay” of the variable delay buffer.
The error calculation portion 15 subtracts the signal OD′(t) representing the hard decision result fed from the hard decision portion 13, from the equalized signal OD(t) supplied from the addition portion 12 so as to obtain an error signal E(t) (E(t)=OD(t)−OD′(t)) that is output. The error signal E(t) from the error calculation portion 15 is sent to the LMS operation portions 16 and 17.
The LMS operation portion 16 performs an LMS operation on the input signal ID(t) and on the error signal E(t) fed from the error calculation portion 15 in order to update the coefficient C0(n) of the FFE 11.
The LMS operation portion 17 performs an LMS operation on the signal OD′(t) representing the hard decision result fed from the hard decision portion 13 and on the error signal E(t) supplied from the error calculation portion 15 in order to update the coefficient C1(n) of the FBE 14.
The channel estimation portion 18 estimates the channel based on the input signal ID(t) and determines the delay amount “delay.” A signal representing the delay amount “delay” determined by the channel estimation portion 18 is sent to the FBE 14. For sparse equalization, it is important accurately to perform channel estimation in order to establish properly the delay amount of the variable delay buffer in the FBE 14.
FIG. 4 is a block diagram showing a structure of the channel estimation portion 18.
The channel estimation portion 18 is made up of a PN correlation calculation portion 31, a correlation peak detection portion 32, a write control portion 33, and a correlation value storage memory 34. The input signal ID(t) is input to the PN correlation calculation portion 31.
The PN correlation calculation portion 31 reproduces the PN sequence and calculates correlation values between the reproduced PN sequence and the input signal ID(t). The correlation values corr(t) obtained by the PN correlation calculation portion 31 are fed to the correlation peak detection portion 32 and correlation value storage memory 34.
The correlation peak detection portion 32 detects a peak of the correlation values corr(t) fed from the PN correlation calculation portion 31, and outputs to the write control portion 33 a peak position flag “pe” representing the peak position.
The correlation value storage memory 34 has three areas formed therein: an area allocated for the dominant wave, an area for pre-echo, and an area for post-echo. The write control portion 33 outputs a write flag “we” so as to write to each of the areas the correlation values corr(t) obtained using the input signals ID(t) before and after a dominant wave position designated by the peak position flag “pe.”
The write control portion 33 outputs the write flag “we” so as to write the correlation values corr(t) corresponding to a time “t” defined astp−X≦t≦tp+Y where, X stands for the size of the pre-echo area, Y for the post-echo area, and “tp” for the time at which a peak position is detected. For example, the fact that the size of the pre-echo area is X indicates that this area can store the correlation values corr(t) obtained using an input signal ID(t) corresponding to the X time.
A delay profile determination portion 35 outputs a read flag “re” so as to read the correlation values corr(t) from the correlation value storage memory 34 and detect the echo position for channel estimation. With the channel estimated, the delay profile determination portion 35 determines the delay amount “delay” accordingly and outputs a signal representing the delay amount “delay” to the FBE 14.
According to the DTMB standard, as explained above, the PN signal is inserted in the start of each frame as a guard interval. Thus correlation values are calculated between the PN sequences and the received signals (input signals ID(t)), and an estimated channel value is obtained from the correlation values.
In connection with the current disclosure, reference may be made to “Determination of Tap Positions for Sparse Equalizers” by Kutz, G., Raphaeli, D.; Communications, IEEE Transactions on, Vol. 55, No. 9, 2007 (Non-patent Document 1).